As the worldwide demand for fuel increases, there is increasing interest in sources other than crude oil for producing diesel fuel. One source of interest is renewable sources, such as vegetable oils and animal fats. A conventional catalytic hydroprocessing process known for converting a renewable feedstock into green diesel fuel may be used as a substitute for the diesel fuel produced from crude oil. The highly exothermic process also supports the possible co-production of propane and other light hydrocarbons, as well as naphtha or green jet fuel. As used herein, the terms “green diesel fuel” and “green jet fuel” refer to fuel produced from renewable sources, in contrast to those produced from crude oil. To produce the green diesel fuel, the renewable feedstock is combined with hydrogen, brought to reaction temperature, and is then sent to a reactor where the renewable feedstock is converted in the presence of a deoxygenation catalyst into a reaction product. The reaction product comprises a liquid fraction and a gaseous fraction. The liquid fraction comprises a hydrocarbon fraction containing normal paraffins. Although this hydrocarbon fraction is useful as a diesel fuel, it has poor cold flow properties. To improve the cold flow properties of the hydrocarbon fraction, the liquid fraction may be contacted with an isomerization catalyst under isomerization conditions to at least partially isomerize the normal paraffins to iso-paraffins. Whether or not isomerization is carried out, the liquid fraction is separated from the gaseous fraction and sent to a fractionation unit to produce the green diesel fuel. The green diesel fuel meets ultra-low sulfur diesel specifications.
Often times, the processing of the renewable feedstock is done intermittently due to availability of renewable feedstock.
However, the deoxygenation catalyst used in the process must remain sulfided when no feed is processed and it is under hydrogen circulation to maintain its performance, but tends to reduce or strip sulfur from the catalyst under those conditions. Separate catalyst sulfide injection systems providing an external source of a sulfiding agent (usually hydrogen sulfide) are typically required to maintain the deoxygenation catalyst in its sulfided form and hydrogen sulfide is an expensive additive and the separate systems increase processing complexity and cost. Hydrogen sulfide may be produced from desulfurizing sulfur-containing petroleum distillate feedstock that refiners typically have in abundant supply.
Since the hydrogen sulfide from processing petroleum distillate feedstock is useful in the processing of renewable feedstock, refiners have tried to co-process petroleum distillate feed and renewable feedstock in a single hydrotreating reactor with the deoxygenation reactions occurring in the same reactor as the desulfurization reaction. However, since the renewable feedstock typically has a higher concentration of chloride compared to the petroleum distillate feed, processing renewable feed stock alone can result in a significant quantity of hydrochloric acid in the reactor effluent as various stages in which water (liquid or steam) may be present. The hydrochloric acid is highly corrosive and can damage equipment and downstream processing units.
Accordingly, it is desirable to provide a method for co-processing a renewable feedstock and a petroleum distillate feedstock to produce a hydrocarbon fraction that will provide a green diesel fuel and maintain the deoxygenation catalyst in a sulfided form, thereby reducing the amount of or eliminating the external sulfiding agent, and at the same time, having such a process that does not result in the severe corrosion due to hydrochloric acid.